The present invention is concerned with waveguides for optical and near optical wavelengths adopted to use nonlinear optical effects of particular kinds and of particular materials.
This application claims priority from Swedish Patent Application No. 9302634-2, filed Aug. 13, 1993, which is incorporated here by reference.
Optical glass fibers are a cheap wavegulde medium that can be exploited for applications in nonlinear optics. Fused quartz, however, has a small nonlinear coefficient as compared with materials such as semiconductors. This has limited considerably the use of fibers in applications such as optical switching, because the optical power of the control signal has to be high in order to cause an appreciable change in the properties of the fiber, see "All-optical Waveguide Switching", G. I. Stegeman, E. M. Wright, Optical and Quantum Electronics 22 (1990), pps. 95-122. This led to the use of discrete components based on LiNbO.sub.3, GaAs and others, see "Integrated Optics in LiNbO.sub.3 : Recent Developments in Devices for Telecommunications", L. Thylen, Journal of Lightwave Tech., 6 (1988) pps. 847-861, "Integrated Optic Devices Based on Nonlinear optical Polymers", E. V. Tomme, P. P van Daele, R. G. Basts, P. E. Lagasse, IEEE Journal of Quantum Electronics vol. 27, Mar. 1991 and "Physical Concepts of Materials for Novel Optoelectronic Device Applications II: Device Physics and Applications", Proceedings SPIE 1362 (1990), and to the search for optical fibers with higher nonlinearity, such as that achieved with semiconductor doped glasses, see "Optical Nonlinearity and Applicatlons of semiconductor-doped Glass Fiber", D. Cotter, B. J. Ainslie, M. G. Butt, S. T. Davey, R. J. Manning, Proceedings CLEO'91, CTuE7, p. 92, and "Efficient non-linear optical fibers and their application", S. Sudo, Itoh, Optical and Quantum Electronics 22 (1990) pps. 187-212, but that are difficult to fabricate. On the other hand, following the development of optical communications the need exists for a simple fiber-based light controlled coupler. With such a device it would be possible to derive an adjustable part of the signal transmitted in a fiber to one or several channels in a transmission network. A fiber based nonlinear optical coupler could also find important applications in logical gates and optical transistors, where a weak signal would control the switching of a higher power signal.
Recent technological developments led to the fabrication of glass fibers with non-cylindrical geometry, and in particular the so-called D-shaped fibers, see "Fabrication and Characterization of D-fibers with a Range of Accurately Controlled Core/Flat Distances", Electronics Letters 22, March 1986. There, light is guided as in a conventional fiber, but the electromagnetic field extends all the way to the glass-air interface. This offers a unique opportunity to make light interact through the evanescent field with any material deposited on the flat surface of the D-shaped fiber. Since this interaction takes place longitudinally along the fiber, this is a particularly favourable geometry that can be exploited in nonlinear optics.
In the patent U.S. Pat. No. 4,557,551 for Dyott a non-linear optical fiber coupler is disclosed having two polarization-maintaining optical fibers of elliptical cross-sections located in parallel at the opposite sides of a central structure. This central structure has a lens-shaped cross-section, that is formed by two oppositely placed arc sections. The central thickness of this middle structure is a few times larger than the diameters of the fibers and many times the diameters of the cores of the fibers. Also, support structures are used for maintaining the fibers in the desired configurations. The middle structure is a single crystal of an electro-optic material having a non-centro-symmetrical crystal structure. The materials mentioned are organic type materials being electric insulators. The physical effect utilized is based on the electro-optical effect, the refractive index of the material being changed by interaction with the electric field of an incoming light wave, the pump wave.
In the European patent application EP-A2 0 164 212 for The Board of Trustees of the Leland Stanford Junior University a fiber optic saturable absorber is disclosed. An optical fiber has a portion of its cladding removed by polishing a slightly curved fiber along plane. A light-absorbing substance having non-linear light-absorbing characteristics, in particular a dye, is applied to the polished surface to make light propagating in the fiber to be absorbed in a controlled way.
In the optic fiber correlator as disclosed in patent U.S. Pat. No. 4,927,223 a D-fiber is used being in contact, at its flattened surface with a material which is non-centro-symmetrical in order to obtain frequency doubling. To the opposite fiber ends laser diodes are connected injecting light of the same wavelengths to the D-fiber. Light emitted by the layer is collected to find the correlation of the signals from the laser diodes.
In an optical device disclosed in the European patent application EP-A1 0 254 509 an optical D-fiber may be provided with a layer on the flat surface, the layer being of a material having a refractive index varying with optical intensity. The free surface of the layer is illuminated by coherent light beams to form a standing wave of a modulated refractive index in the layer resulting in a modulated index grating.